Support employer rebranding

[jace]

Employers may rebrand during the course of their existence. While Hasjob allows a job to be edited and all details changed, there is no support for (a) changing the email address and associated domain, or (b) merging domains where users have used both old and new domains.

HasGeek itself is affected by problem b with hasgeek.in and hasgeek.com domains both used on Hasjob.

This is a separate problem from #157 that allows changing email within domain.

We do not have a workflow for automated verification of domain changes, so this should likely be part of a support backend.